1. Field of the Invention
This invention relates to a solid fuel burning furnace, and in particular to such a furnace of the downdraft type which directs a controlled jet of primary air over the bottom surface of the fuel while operating in a passive mode, that is, without the addition of mechanical air circulation to improve combustion efficiency.
In the optimum condition, the release of stored energy in the combustion of solid fuel should be accomplished in a controlled manner with consequent reduction or virtual elimination of pollutant discharge to the atmosphere. The complexity of combustion is demonstratable through subtle variation of reaction conditions and thus combustion responds to even dimensional variations of an optimized combustion environment.
Solid cellulose fuels decompose or burn in the presence of sufficient heat to produce a charcoal or solid fuel component and a volatile or gaseous fuel component. It is this volatile fuel which requires satisfaction of several specific criteria to accomplish complete combustion. A combustible mixture which is a proper balance of volatile fuel and oxygen must be delivered to the combustion environment in which provisions are made to ensure the presence of high ignition temperature and sufficient turbulence to ensure a fuel and oxygen contact. The temperature of the combustion environment must be sufficient for the particular fuel being burned, and, in the case of wood, eleven hundred degrees (1100.degree.) Fahrenheit is considered minimum.
Some state of the art devices utilize mechanically assisted components such as fans to induce high velocity air into the combustion chamber to generate high temperature and turbulence. Although these devices may achieve improved combustion efficiency and minimized emissions to the atmosphere, certain disadvantages are inherent in these active systems.
The high temperatures and rates of heat production of mechanically aspirated systems often require the practical disadvantages of expensive combustion chambers, control mechanisms, active components and heat storage elements. These cost disadvantages have thereby minimized their in service use and appeal.
Updraft furnaces whose primary combustion products exit at or near the top of the combustion chamber are common due to their simple design and low initial cost. They tend however to have the in service disadvantages of low thermal efficiencies and high pollutant discharge to the atmosphere.
Downdraft furnaces whose outlets are at or near the bottom of the combustion chamber offer the advantages of higher thermal efficiencies and lower emissions to the atmosphere if appropriate considerations for fuel loading, temperature generation and oxygenation are made.
The disclosed invention has been demonstrated to accomplish an unusually high level of clean combustion while burning various cellulose materials generally with no visible atmospheric emissions and accomplished through non-obvious aerodynamic and structural features.
2. Description of the Prior Art
A relevant downdraft stove of the type here under consideration is disclosed in U.S. Pat. No. 2,564,713 dated Aug. 21, 1951. This patent discloses a stove designed specifically for coal burning with provisions for coal feeding zones, secondary combustion and a radiation chamber. This invention and the very similar U.S. Pat. No. 2,433,036 fail however to anticipate and address the characteristics of longitudinal fuels such as logs or sticks which by their nature alter the flow of air along and around them. The administration of logs to either of these prior art devices would thereby alter their function significantly from that of operation with only coal. The loose filling characteristics of coal and the structural characteristics of logs should therefore be recognized in order to optimize the downdraft function.
In U.S. Pat. No. 2,564,713 a diffused air flow is generally applied to the base of the fuel loading with a trajectory perpendicular to the surface on which fuel would lie. This has several disadvantages in the presence of log type fuels. First, logs loaded upon the inclined surfaces therein described would be stimulated to burn upwards through the log layers and thereby involving more fuel than just logs upon the bottom layer. Secondly this trajectory in combination with the inclined support surfaces when logs are placed lengthwise on the incline will tend to guide the flame by convection upwards and to the front away from rather than to the secondary combustion chamber as disclosed. This reversal thereby exposes more fuel to premature burn conditions resulting in decreased combustion efficiency and increased atmospheric emissions. This device also places the secondary combustion chamber inlet above the bottom of the primary combustion chamber. This further encourages the flow upwards through the fuel loading as occurs in updraft appliances rather than downward and aggravating the premature burn problem. In this location gaseous fuels are drawn away from rather than across or through the coals layer at the bottom. This diminished coals contact reduces flame temperature entering the secondary combustion chamber thereby reducing secondary combustion efficiency. Additionally neither device specifies means of guiding primary combustion air with minimal flow obstruction toward the secondary combustion chamber inlet.
It is these design deficiencies and more which the present invention and its embodiments seek to correct as will be apparent in the following summary of the invention.